In radio communications systems, speech or other user data is transmitted between radio terminals, in particular between a base station of a network and a mobile station, by using Traffic Channels TCH. In order to enable an efficient voice transmission, several aspects related to the transmission, like the allocation of radio resources and handovers, have to be managed in co-operation between the network and respective mobile stations. For such a management, signalling messages must be conveyed over the air interface in addition to user data between the network and a mobile station in both direction. The signalling messages can be transmitted independently, as long as no user data is transmitted.
When traffic is ongoing, the signalling messages are transmitted in Associated Control Channels ACCH.
It is known from the state of the art to use two different kinds of ACCHs, Slow Associated Control Channels (SACCH) and Fast Associated Control Channels (FACCH). FACCHs enable a faster transmission of signalling messages than SACCHs. SACCHs are therefore used for non-urgent procedures, mainly for the transmission of the radio measurement data needed for handover decisions. FACCHs (or main DCCHs) are involved in delay sensitive mechanisms such as handover. The use of SACCHs and FACCHs is described in more detail e.g. in “GSM 03.64: Digital cellular telecommunications system (Phase 2+), Overall description of the GPRS radio interface; Stage 2”, “GSM 04.18: Digital cellular telecommunications system (Phase 2+), Mobile radio interface layer 3 specification, Radio Resource Control Protocol”, “GSM 04.60: Digital cellular telecommunications system (Phase 2+), General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol”, “GSM 05.08: Digital cellular telecommunications system (Phase 2+), Radio Subsystem Link Control” or “GSM 05.10: Digital cellular telecommunications system (Phase 2+), Radio Subsystem Synchronization”.
Currently, speech is transmitted on Traffic Channels using only GMSK (Gaussian-filtered Minimum-Shift Keying) modulation. In GERAN (GSM/EDGE RAN; GSM: Global System for Mobile communication, EDGE: Enhanced Data rates for GSM Evolution, RAN: Radio Access Network), speech will also be transmitted using 8PSK (Phase Shift Keying) modulation. Therefore, FACCHs will be either GMSK modulated if the co-transmitted speech is GMSK modulated and 8PSK modulated if the co-transmitted speech is 8PSK modulated. SACCH/TF (Slow, TCH/Full rate-Associated Control Channels) will be GMSK modulated for both, 8PSK and GMSK modulated speech, in full rate and half rate transmission mode.
Originally, SACCH and FACCH were designed for Full Rate speech Traffic Channels (TCH/FR) and later on for Enhanced Full Rate speech Traffic Channels (TCH/EFR) used in GSM. When the Adaptive Multi Rate (AMR) speech codecs were introduced for GSM in “GSM 05.03: Digital cellular telecommunications system (Phase 2+), Channel Coding”, the same ACCHs were re-used.
The different AMR speech codecs aim at adapting the coding scheme to the link level quality. In a bad environment, a low speech codec with a low coding rate will be used, whereas in a good environment, a higher speech codec can be used with a higher coding rate. AMR thereby allows worse channel conditions for user data transmission as compared to traditional TCH/FR and TCH/EFR. As mentioned above, the channel coding of the ACCHs, however, was not adapted accordingly. As a consequence, while AMR allows data transmission under worse conditions, the block error rate of the ACCHs increases under such conditions. Therefore, more retransmissions are needed for the ACCHs and the transmission delay is increased, reducing the overall system performance.